1. Field of the Invention
The present invention relates generally to a position measurement technique, and more particularly to a capacitance sensation unit of plane position measurement device.
2. Description of the Related Art
U.S. Pat. No. RE27436 discloses a technique related to flat stepping motor. The basic structure of the flat stepping motor is mainly composed of a flat steel plate and a mover movable on the surface of the steel plate. The mover is able to interact with the magnetic field produced by multiple stator teeth two-dimensionally arranged on the surface of the steel plate in a checkerboard pattern. Accordingly, the mover can quickly and precisely move on the surface of the steel plate. However, such flat stepping motor necessitates a laser interferometer for performing position measurement. The laser interferometer is quite expensive and complicated so that the substantial application range of the flat stepping motor is greatly limited.
Many conventional sensation techniques have been disclosed and used for performing practical sensation and measurement of the position of the mover of the flat motor. For example, U.S. Pat. No. 6,476,601 discloses a compensation magnetic sensor on the basis of Hall sensation. Such sensor is too sensitive to the residual magnetization affection of the stator teeth. As a result, the practical application of such sensor is limited.
U.S. Pat. No. 6,175,169 discloses an improved electromagnetic sensor, which is integrated with the mover. The electromagnetic sensor has submicron-level sensation ability. However, the precision of such electromagnetic sensor is lowered with the crossmodulation effect, affection of magnetic flux of the mover and the defects of the stator teeth due to small-scale sensation. Therefore, such electromagnetic sensor is still not perfect.
U.S. Pat. No. 5,818,039 discloses a sensation device based on optical sensation technique. Such sensation device is able to detect the change of position through fluorescence. However, such sensation device has a quite large volume so that it is impossible to integrally arrange the sensation device in the mover. This limits the application of such sensation device in the field of flat motor. Also, such sensation device can hardly provide uniform dyeing concentration and is unable to remove the noise in the position signal. As a result, such sensation device cannot perform precise position measurement.
In the above different sensation techniques, the technical essences are all limited. Therefore, none of the above sensation techniques can provide an optimal position measurement for the flat motor. Thanks to the development of flat motor manufacturing technique, the flat motor can have a stable air gap. The stable air gap is a basis for the capacitance measurement technique. Accordingly, the capacitance measurement technique with nano-definition precision and insensitivity to magnetic flux has become a most often used technique for plane position measurement.
Concerning the capacitance measurement technique, U.S. Pat. No. 6,492,911 discloses a sensor based on rotational and linear capacitances. Such sensor can modulate the position through electrodes in a specific configuration. However, due to the specialness of the configuration, such sensor cannot be applied to the plane linear motor.
In addition, U.S. Pat. No. 4,893,071 discloses a capacitance sensation technique performing position measurement on the basis of the stator teeth. However, in such capacitance sensation technique, the harmonic wave distortion of the position sensation signal will greatly lower the precision. Also, the technique for rectifying and adjusting the relative motional position of the sensor is too complicated to integrate the sensor on the armature of the mover.